Pressure-contact semiconductor device capsules 10 are often used in electricity transmission and distribution infrastructures, and typically include a semiconductor assembly 12 mounted between two opposed poles 14,16, as shown in FIG. 1. The semiconductor assembly 12 and poles 14,16 are usually provided within an insulating (e.g. ceramic) enclosure (not shown).
In use, the capsule 10 is subject to a compressive mechanical clamping force to ensure good thermal, electrical and mechanical contact between the semiconductor assembly 12 and the poles 14,16. This force is usually applied through heat sinks 18,20 and/or solid surface plates placed either side of the capsule 10.
During operation, the semiconductor assembly 12 generates heat, thereby heating the lower surface of the top pole 14 and the upper surface of the bottom pole 16. The upper surface of the top pole 14 and the lower surface of the bottom pole 16 are cooled by the heat sinks 18,20. This creates a thermal gradient and, as a result, causes the top and bottom poles 14,16, and other parts of the assembly, to bend in opposite directions, as illustrated in FIG. 2.
This bending alters the thermal, electrical and mechanical contact between the semiconductor assembly 12 and the poles 14,16. This is the case even when the junction temperature is limited to 125° C., as in the case of silicon-based semiconductor assemblies. When junction temperatures of 250° C. are permissible, as with semiconductor assemblies based on silicon carbide, higher heat flux results in greater thermal deformation.
In circumstances where the semiconductor assembly includes a single, and often large diameter wafer, the metallization of the semiconductor wafer, typically evaporated or sputtered aluminium, is conformable and is usually of sufficient thickness to compensate for the effects of bending. Contact between the wafer and the poles may be improved by interposing molybdenum or tungsten plates of intermediate expansion coefficient. In circumstances where the semiconductor assembly includes a large diameter wafer, the provision of molybdenum or tungsten plates also lessens the likelihood of the wafer cracking.
However, in circumstances where the semiconductor assembly includes a multiplicity of separate chips, bending results in poor distribution of thermal, electrical and mechanical contact, particularly near the circumference of the capsule. This causes poor equality of contact between the constituent chips, and results in electrical duty being concentrated on the chips located at the centre of the capsule. The chips located at the centre of the capsule may therefore overheat and fail.
Contact between each of the chips and the poles may be improved by locating a spring on one side of each of the chips to provide a spring bias. However, this means that heat flux can only flow effectively from one side of each of the chips since heat transfer through the springs is very much reduced compared to standard contact methods.
Contact between each of the chips and the poles may also be improved by increasing the compressive mechanical clamping force in order to press flat the distortion caused by the thermo-mechanical bending of each pole. However, it is known that the magnitude of the compressive mechanical clamping force is limited since it is important not to damage the semiconductor chips. If the clamping force exceeds a predetermined value, the structure of the semiconductor chips will be damaged.
It is also known from analysis and modelling that the magnitude of the compressive mechanical clamping force required to press flat the distortion caused by the thermo-mechanical bending of each pole is dependent on, and increases markedly with, the thickness of the pole. However, it is important to ensure that the poles are sufficiently thick that the depth of the capsule is not less than approximately 25 mm (and preferably somewhat thicker). This ensures that the insulation provided by the ceramic enclosure around the capsule has adequate jump and creepage distance. The creepage distance is provided by forming the ceramic with sufficiently deep sheds. This thickness requirement translates into the need to use poles which have an overall thickness of at least 10 mm.